Is it normal to feed a mouse to a cat?

Before you judge the ethics, morals and depravity of the question, please read the 'whole' story.

I own a Maine Coon cat named Cassie who is 18 years old, has lived a great life, has traveled with me up and down the CA coast, and who has been an avid hunter. It wasn't until recently, I noticed her slowing down. Now many people see this in the much earlier years in their pets, but she had never been that way a few days ago, and I got worried. Her playfulness was gone, and she slept a lot more. I thought maybe old age was finally catching up with her, so I began keeping her inside for fear of cars or wild animals. This was a big mistake on my part. She got even more lethargic and stopped eating and grew thin. Naturally I panicked and took her to the vet. The results were that her body was healthy, she was slowing down, but that was normal for her age, but the real problem was she had a form of depression. She said any change in the household or my routine can disrupt a cat's life, and cause depression. I told her I've been keeping her inside more often, and she recommended I test out letting her out more. If she still won't eat, take her back immediately.

So I did, and it worked, but only a little. She nibbled her food, but never ate more than a teaspoon a day and continued getting thinner. She was still depressed, and she'd watch her kittens playing outside and hunting instead of joinng them like she used to. But when her eldest brought her a half live mouse, I saw Cassie jump up, play with it, then kill and eat it whole. For that frame of time, the old Cassie had returned. And it gave me an idea. I went to a reptile shop where they sold feeder mice, and bought one. I was hesitant to give her it at first, but if it worked and got her to eat...maybe it was worth it.

Well I gave the mouse to her, it wasn't injured in anyway and had a chance to escape, Cassie jumped up, and caught it, then ate it. That night, I also saw her eat some of her normal food. This cycle continued on for a week, and I noticed fat returning to her body. Her rips were no longer visible anymore, she was more active and played with her kittens. I almost cried, she was healthy again. I went back to vet to make sure my suspicions were true, and her results were very positive. I told her about the mice diet I gave Cassie, and while she said it was an unorthodox way, at least it worked. So seeing Cassie eat her normal food, I stopped the mouse diet. Then the symptoms came back again the next week. I was at a loss of what to do.

I don't want Cassie to die, she won't play with toys or eat chopped raw meat. She wants live prey, she likes the movement and the meal afterword, and I've been getting a guilt trip from my sister about how inhumane it is. So, is it normal to feed a cat a mouse, if it prevents the cat from wasting away?